ABSTRACT Staphylococcus aureus is an invasive human pathogen, causing soft tissue, wound, lung, skeletal and bloodstream infections in community- and hospital-settings. Infection with antibiotic-resistant strains, designated methicillin-resistant S. aureus (MRSA), is associated with treatment failure and poor disease outcomes. MRSA and methicillin-sensitive (MSSA) strains are frequent causes of infectious disease morbidity and mortality in the United States. Hallmark of all S. aureus disease is its recurrence and the inability of infected hosts to establish protective immunity. Persistent colonization of the human nasopharynx and gastrointestinal tract is a key risk factor for invasive S. aureus disease. The development of a staphylococcal vaccine that can block colonization and prevent invasive disease represents an urgent, unmet clinical need. We have conducted studies in S. aureus infected humans and analyzed persistent colonization and invasive disease in animals. Our data demonstrate that staphylococcal protein A (SpA), a sortase-anchored surface protein, prevents development of protective immunity during S. aureus colonization and infection. When anchored to the bacterial cell wall, SpA binds to the Fc? domain of IgG, blocking the effector functions of antibodies and inhibiting opsonophagocytic killing (OPK) of bacteria. Staphylococci also release peptidoglycan- linked SpA into host tissues, thereby crosslinking VH3 idiotype B cell receptors (BCRs) and promoting non- productive B cell proliferation as well as secretion of non-protective VH3 idiotype antibodies. Another sortase- anchored product, adenosine synthase A (AdsA), converts ATP to adenosine and neutrophil NET DNA to deoxyadenosine, triggering apoptosis of macrophages and preventing OPK of S. aureus. Immunization with non-toxigenic SpA, which cannot bind IgG Fc? or crosslink BCRs, elicits SpA-neutralizing antibodies that promote OPK and block SpA-mediated B cell superantigen activity (BCSA) during S. aureus colonization and invasive disease. SpA-neutralizing antibodies promote development of diverse pathogen-specific IgG responses that block S. aureus colonization and prevent invasive disease. In this proposal, we seek to characterize a non-toxigenic SpA variant devoid of BCSA in order to develop a human vaccine that blocks S. aureus colonization and invasive disease. Further, we will explore the molecular and cellular mechanisms whereby peptidoglycan-linked SpA activates B cells and CD4+ T cells to divert adaptive immune responses during infection. Last, we will identify host determinants required for AdsA-mediated immune evasion of S. aureus.